fix mysql import error. uninitialized constant.

The following error occurred import in MySQL.
uninitialized constant ActiveRecord::Import::ValueSetsBytesParser
Is it a typo?

Did using only in this file.
https://github.com/zdennis/activerecord-import/blob/27139cfa7902df0af2e9415ec22551e1f111a83e/lib/activerecord-import/adapters/mysql_adapter.rb#L39